Electro-conductive B4C-TiB2 composite ceramic and preparation method thereof

ABSTRACT

An electro-conductive B4C—TiB2 has a microstructure in which large B4C grains are coated by small TiB2 grains. The composite ceramic includes 10˜30% by volume of TiB2. A method for preparing the electro-conductive B4C—TiB2 composite ceramic includes: (1) weighing B4C, TiC, and amorphous B powder; (2) mixing evenly and drying thoroughly the powders; and (3) loading the mixed powder into a graphite mold; and placing the graphite mold in a spark plasma sintering furnace for sintering under vacuum, where the sintering is performed at 2000° C. and 50 MPa for 5˜20 min.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from Chinese Patent Application No. 202210485924.X, filed on May 6, 2022. The content of the aforementioned application, including any intervening amendments thereto, is incorporated herein by reference in its entirety.

TECHNICAL FIELD

This application relates to ceramic materials, and more particularly to an electro-conductive B₄C—TiB₂ composite ceramic and a preparation method thereof.

BACKGROUND

B₄C has been widely used in bulletproof armor, wear-resistant devices, aerospace and nuclear industries because of its high hardness, low density, good chemical stability, and large neutron-absorption cross section. Due to the presence of covalent bonds, B₄C has poor sintering performance, which makes it difficult to achieve densification. Furthermore, the low fracture toughness of monolithic B₄C ceramic leads to poor machining efficiency and accuracy and high cost, which limit the application of the monolithic B₄C ceramic.

The introduction of a second phase TiB₂ into the B₄C matrix to form B₄C—TiB₂ composite ceramics can not only significantly improve the sintering performance and mechanical properties of B₄C ceramics, but also greatly reduce the electrical resistivity thereby meeting the requirements of electrical discharge machining (EDM). Usually, a material with lower electrical resistivity is more suitable for the EDM processing. Therefore, a key to improving the EDM processing performance of B₄C—TiB₂ composite ceramics is reducing the electrical resistivity. The most effective way to reduce the electrical resistivity is increasing the conductive phase content in composite ceramics. However, the increased content of TiB₂ conductive phase in the B₄C—TiB₂ composite ceramics will bring the following problems: (1) increasing the overall density of the composite ceramics (the density of TiB₂ is about 1.8 times that of B₄C); (2) reducing the hardness of composite ceramics (the hardness of TiB₂ is lower than that of B₄C); and (3) increasing the raw material cost. The above problems limit the application of B₄C—TiB₂ composite ceramic in lightweight armor. In conventional methods, a lower electrical resistivity of B₄C—TiB₂ composite ceramics is achieved mainly at the expense of other properties or cost.

SUMMARY

In view of the deficiencies in the prior art, this application provides an electro-conductive B₄C—TiB₂ composite ceramic having a high electrical conductivity at low TiB₂ content and a preparation method thereof In the microstructure of the B₄C—TiB₂ composite ceramic provided herein, the large B₄C grains are coated by small TiB₂ grains.

Technical solutions of this application are described as follows.

In a first aspect, this application provides an electro-conductive B₄C—TiB₂ composite ceramic, wherein the electro-conductive B₄C—TiB₂ composite ceramic has a microstructure in which B₄C grains are coated by TiB₂ grains; a grain size of the B₄C grains is much larger than that of the TiB₂ grains; and a TiB₂ volume percentage in the electro-conductive B₄C—TiB₂ composite ceramic is 10˜30%;

wherein the electro-conductive B₄C—TiB₂ composite ceramic is prepared through steps of:

-   -   (1) weighing a B₄C powder, a TiC powder, and an amorphous B         powder according to a preset weight ratio;     -   wherein a particle size of the B₄C powder is 3.0˜20.0 μm; a         particle size of the TiC powder is 0.05˜3.0 μm; a particle size         of the amorphous B powder is 0.5˜1.0 μm; a molar ratio of TiC to         B is 1:6.6; and a weight ratio of the B₄C powder to the TiC         powder to the amorphous B powder is 21-69:14-36:16-43;     -   (2) mixing the B₄C powder, the TiC powder and the amorphous B         powder evenly followed by drying to obtain a mixed powder; and     -   (3) loading the mixed powder into a graphite mold; and         transferring the graphite mold to a spark plasma sintering (SPS)         furnace followed by sintering under vacuum.

In a second aspect, this application further provides a method for preparing the electro-conductive B₄C—TiB₂ composite ceramic, including:

-   -   (1) weighing a B₄C powder, a TiC powder, and an amorphous B         powder according to a preset weight ratio;     -   wherein a particle size of the B₄C powder is 3.0˜20.0 μm; a         particle size of the TiC powder is 0.05˜3.0 μm; and a particle         size of the amorphous B powder is 0.5˜1.0 μm; a molar ratio of         TiC to B is 1:6.6; and a weight ratio of the B₄C powder to the         TiC powder to the amorphous B powder is 21-69:14-36:16-43;     -   (2) mixing the B₄C powder, the TiC powder and the amorphous B         powder evenly followed by drying to obtain a mixed powder; and     -   (3) loading the mixed powder into a graphite mold; and         transferring the graphite mold to a spark plasma sintering (SPS)         furnace followed by sintering under vacuum.

In an embodiment, the sintering is performed at 2000° C. and 50 MPa for 5˜20 min.

Compared to the prior art, this application has the following beneficial effects.

-   -   (1) A microstructure, in which the large B₄C grains are coated         by the small TiB₂ grains, is constructed, which is conducive to         the formation and improvement of the electro-conductive network.     -   (2) The spark plasma sintering (SPS) technology can effectively         inhibit the growth of TiB₂ grains, which is conducive to         improving the electrical properties of the B₄C—TiB₂ composite         ceramic.     -   (3) In the case of the same TiB₂ content, the B₄C—TiB₂ composite         ceramic prepared herein has higher electrical conductivity.     -   (4) In the case of the same resistivity, the B₄C—TiB₂ composite         ceramic provided herein has lower TiB₂ content.     -   (5) The larger the particle size of B₄C, the higher the         conductivity of the composite ceramic, which helps to reduce the         cost of raw materials.     -   (6) Compared with the traditional method, the mechanical         properties of the B₄C—TiB₂ composite ceramic prepared in the         present application are comparable or better.     -   (7) The preparation process of the present application is         simple, including only mixing and sintering of raw powders,         without any complex operations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a fabrication principle of an electro-conductive B₄C—TiB₂ composite ceramic according to an embodiment of the present disclosure;

FIGS. 2 a-2 c show microstructure changes of a polished surface of a B₄C—TiB₂ composite ceramic prepared in Example 1 of the present disclosure; and

FIG. 3 is a microstructure diagram of a fracture surface of the B₄C—TiB₂ composite ceramic prepared in Example 1 of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

The technical solutions of the present disclosure will be further described in detail below in conjunction with the accompanying drawings and embodiments, but the embodiments below are not intended to limit the disclosure.

Example 1

5.03 g of B₄C powder with a particle size of 10.3 μm, 1.83 g of TiC powder with a particle size of 0.05 and 2.19 g of amorphous B powder with a particle size of 0.9 μm were weighed, mixed uniformly, and dried to obtain a mixed powder. The mixed powder was loaded into a graphite mold, subjected to sintering in a spark plasma sintering furnace under vacuum at 2000° C. and 50 MPa for 16 min, and cooled naturally to obtain an electro-conductive B₄C-15 vol % TiB₂ composite ceramic. As demonstrated by the performance test, the prepared B₄C-15 vol % TiB₂ composite ceramic had a relative density of 98.7%, a three-point flexural strength of 676 MPa, a Vickers hardness of 29.0 GPa, a fracture toughness of 5.3 MPa·m^(1/2), and an electrical conductivity of 2.8×10⁴ S/m.

A B₄C-15 vol % TiB₂ composite ceramic prepared from 10.3 μm B₄C powder and 2.5 μm TiB₂ powder through the same mixing and sintering process was used as comparison. As demonstrated by the performance test, the obtained B₄C-15 vol % TiB₂ composite ceramic had a relative density of 95.3%, a three-point flexural strength of 552 MPa, a Vickers hardness of 27.5 GPa, a fracture toughness of 4.4 MPa·m^(1/2), and an electrical conductivity of 4.3×10³ S/m.

FIG. 1 illustrated the construction of the B₄C—TiB₂ composite ceramic of the present disclosure, where (a) mixing of raw materials B₄C, TiC, and B powder; (b) during the sintering process, TiC and B first underwent an in-situ reaction to form B₄C—TiB₂ ultrafine composite powder; (c) large B₄C particles selectively absorbed ultrafine B₄C particles in the B₄C—TiB₂ composite powder to experience grain growth; and (d) small TiB₂ grains are distributed around the large B₄C grains to form an enveloped microstructure.

FIGS. 2 a-2 c show microstructure change of the polished surface of the B₄C—TiB₂ composite ceramic prepared in Example 1, where FIG. 2 a : in the early stage of the sintering process, the large B₄C particles and the in-situ formed B₄C—TiB₂ ultrafine composite powder coexisted; FIG. 2 b : with the extension of the sintering time, the large B₄C particles selectively absorbed the ultrafine B₄C particles in the B₄C—TiB₂ composite powder to experience grain growth; and FIG. 2 c : after holding for a certain time, the ultrafine B₄C particles in the B₄C—TiB₂ composite powder were completely absorbed by the large B₄C particles, and the small TiB₂ grains are distributed around the large B₄C grains to form an enveloped microstructure. During this process, TiB₂ also underwent grain growth. However, due to the small initial grain size and limited growth, the TiB₂ grain was still much smaller relative to the B₄C grain.

As shown in FIG. 3 , the small TiB₂ grains were distributed around the large B₄C grains.

Example 2

5.99 g of B₄C powder with a particle size of 3.1 μm, 1.22 g of TiC powder with a particle size of 0.8 μm, and 1.46 g of amorphous B powder with a particle size of 0.9 μm were weighed, mixed uniformly, and dried to obtain a mixed powder. The mixed powder was loaded into a graphite mold, subjected to sintering in a spark plasma sintering furnace under vacuum at 2000° C. and 50 MPa for 16 min, and cooled naturally to obtain an electro-conductive B₄C-15 vol % TiB₂ composite ceramic. As demonstrated by the performance test, the prepared B₄C-15 vol % TiB₂ composite ceramic had a relative density of 99.5%, a three-point flexural strength of 780 MPa, a Vickers hardness of 31.8 GPa, a fracture toughness of 5.8 MPa·m^(1/2), and an electrical conductivity of 3.3×10³ S/m.

B₄C-15 vol % TiB₂ composite ceramic prepared from 3.1 μm B₄C powder and 2.5 μm TiB₂ powder through the same mixing and sintering method was used as comparison. As demonstrated by the performance test, the obtained B₄C-15 vol % TiB₂ composite ceramic had a relative density of 98.65%, a three-point flexural strength of 638 MPa, a Vickers hardness of 29.2 GPa, a fracture toughness of 4.9 MPa·m^(1/2), and an electrical conductivity of 2.1×10³ S/m.

Example 3

4.07 g of B₄C powder with a particle size of 10.3 μm, 2.45 g of TiC powder with a particle size of 0.8 and 2.91 g of amorphous B powder with a particle size of 0.9 μm were weighed, mixed uniformly, and dried to obtain a mixed powder. The mixed powder was loaded into a graphite mold, subjected to sintering in a spark plasma sintering furnace under vacuum at 2000° C. and 50 MPa for 16 min, and cooled naturally to obtain an electro-conductive B₄C-20 vol % TiB₂ composite ceramics. As demonstrated by the performance test, the prepared B₄C-20 vol % TiB₂ composite ceramic had a relative density of 98.1%, a three-point flexural strength of 701 MPa, a Vickers hardness of 28.5 GPa, a fracture toughness of 6.2 MPa·m^(1/2), and an electrical conductivity of 6.9×10⁴ S/m.

B₄C-20 vol % TiB₂ composite ceramic prepared from 10.3 μm B₄C powder and 2.50 μm TiB₂ powder by the same mixing and sintering method was used as comparison. As demonstrated by the performance test the obtained B₄C-20 vol % TiB₂ composite ceramic had a relative density of 93.5%, a three-point flexural strength of 587 MPa, a Vickers hardness of 27.6 GPa, a fracture toughness of 5.1 MPa·m^(1/2), and an electrical conductivity of 1.6×10⁴ S/m. 

What is claimed is:
 1. A method for preparing an electro-conductive B₄C—TiB₂ composite ceramic, comprising: mixing a B₄C powder, a TiC powder and an amorphous B powder according to a preset weight ratio to produce a powder mixture; wherein a particle size of the B₄C powder is 3.0˜20.0 μm; a particle size of the TiC powder is 0.05˜3.0 μm; and a particle size of the amorphous B powder is 0.5˜1.0 μm; drying the powder mixture; loading dried powder mixture into a graphite mold; and transferring the graphite mold to a spark plasma sintering (SPS) furnace followed by sintering under vacuum.
 2. The method of claim 1, wherein the sintering is performed at 2000° C. and 50 MPa for 5˜20 min.
 3. The method of claim 1, wherein a molar ratio of TiC to B is 1:6.6; and a weight ratio of the B₄C powder to the TiC powder to the amorphous B powder is 21-69:14-36:16-43. 